The Pitt Men's Study (PMS) was initiated in 1983 as one of the four centers participating in the prospective Multicenter AIDS Cohort Study (MACS) examining the natural history of HIV infection in homosexual men. The PMS epidemiologic work includes monitoring seroprevalence and seroconversion to HIV in our cohort, assessing health care utilization and health care services available to our cohort, assessing sexual behavior and secular changes in high risk behavior in relationship to HIV seroconversion, risk factors for HIV infection, medication and drug use, predictors of AIDS, and determining the symptomatology associated with both acute and chronic HIV infection. Neuropsychological research primarily assesses the risk for developing neurologic impairments amongst seropositive and seronegative men, and relationships to immune system function. Immunological studies focus primarily on cell-mediated immune reactions to HIV, including cytotoxic T lymphocyte responses, natural killer cell and lymphokine-activated killer cell activity, T lymphocyte blastogenesis and HIV-dendritic cell interactions. Virological work includes comparison of HIV isolates from asymptomatic men and AIDS patients by various techniques and quantitation of HIV DNA and RNA by the polymerase chain reaction. These studies have been integrated into the new MACS initiatives on the viral immunopathogenesis of HIV infection in various subpopulations of our cohort (i.e., nonprogressors, intermediate progressors, rapid progressors). The PMS provides a unique opportunity to evaluate the occurrence of malignancies in HIV infected persons and those at high risk of HIV infection. In addition, autopsies are obtained on members of the cohort. The tissues are stored in a central tissue and storage facility located at the PMS. Collectively, the MACS continues to be a productive cohort study. The baseline risk factors for HIV infection have been described. Numerous studies are at various stages of investigation to delineate further the epidemiology, immunology and molecular virology of the natural history of HIV infection in homosexual men.